Interferometric imaging systems based on laser illumination of an object (employing an optical system of the type depicted in FIG. 1) are well known to provide three-dimensional information about the object being inspected. Two existing methods utilize single wavelength laser interferometry and two wavelength laser interferometry.
Single wavelength interferometry records an interferogram of the surface of an object being inspected. A measure of the height difference (hereinafter also referred to as the range or depth) of points on the object from a flat reference surface may be ascertained in the single wavelength system by calculating the difference in path lengths at each point between the object and reference beams. However, the phase measured by the system is a modulo 2.pi. quantity and thus gives rise to an ambiguity whereby the same phase value .phi. results for pixels that have distance values that differ by integral numbers of wavelengths. While spatial unwrapping algorithms have been used to determine the depth profiles of smooth objects, this method is ineffective for surfaces that have height steps that are larger than one wavelength.
Two-wavelength interferometry has been proposed to extend the range ambiguity interval. U.S. Pat. No. 4,832,489, issued to Wyant and Creath, discloses a method of recovering the optical path difference from the phase difference for two images collected at two wavelengths. Specifically, Wyant and Creath discloses defining a phase difference quantity EQU .PHI..sub.eq =.phi..sub.1 -.phi..sub.2 ( 1)
Where .phi..sub.1 and .phi..sub.2 are the interferometric images recorded at wavelengths .lambda..sub.1 and .lambda..sub.2 respectively. The optical path difference is then found via ##EQU1## The ambiguity interval for this imaging method is .lambda..sub.eq.
With this two wavelength method, the ambiguity level varies inversely with the wavelength separation. For large range ambiguity, one would typically employ closely spaced wavelengths in this system. However, this reduces range resolution, which can only be increased in the two wavelength regime by broadly spacing the wavelengths.
Thus, while single and two-wavelength interferometric imaging methods may be utilized to measure the profiles of relatively smooth objects such as optical components, these methods have limitations which prevent inspection of objects with a wide variety of surface reflectivity, as well as inspecting objects with large depth extents at fine resolution.
These inventors have proposed the use of multi-wavelength (i.e. greater than two wavelength) laser illumination interferometry which overcomes the limitations of the above-described methods by employing greater than two wavelengths, with some closely spaced to provide a large ambiguity level, and with the spacing of the extreme wavelengths of the series large enough to give fine resolution.
One problem encountered with performing three-dimensional interferometric imaging using these laser illumination techniques is phase errors which may result from insufficient knowledge of the laser frequency values, differences in material dispersion properties caused by differences in the number and types of optical components in the paths of the object and reference beams, and relative motion (drift) between the object and reference beams during the data collection time interval. Left uncorrected, these phase errors may result in an aberrated range profile, thereby impairing the ability to determine the location of the peak of the range profile, thus adversely affecting range (or depth) resolution accuracy. Methods for limiting the effect of these phase errors include the use of an optical system which is adjunct to the three-dimensional imaging system to explicitly measure the laser frequencies, use of imaging system configurations in which the path lengths of the object and reference beams are matched (such as when there are differences in the material dispersion properties for the object and reference beam paths), and attempting to maintain mechanical rigidity over the data collection time interval. These solutions, generally directed to the increased stabilization of the optical system to limit the phase errors, are typically costly and cumbersome since they require additional hardware components and/or impose additional constraints on the mechanical configuration which may limit the capabilities of the system.